The invention generally relates to method for manufacturing porous articles having a predetermined structure and properties. As such, the invention is well suited for producing metallic and nonmetallic materials having open or closed pore structures of predetermined sizes and shapes.
A number of techniques have been proposed for manufacturing porous articles. The most widely used techniques are those based on the sintering of powders, chips, fibers, nets, channeled plates and combinations thereof. Also known in the art are processes using a slurry which is foamed and subsequently baked and sintered. Other processes known in the art include slip forming or slurry casting techniques. In slip forming, porous cellular materials are produced by pouring slip into a porous mold whose contents are subsequently dried and baked to remove the slip fluid and leave behind a powder compact. Another method which is presently used is based on the depositing of a metal onto an organic substrate, such as polyurethane, which is then removed by thermal-decomposition.
The nature of the present invention includes features more closely related to processes used for casting metals, including melting a base metal or alloy and subsequently solidifying the melt to form the required composite.
In the field of metal casting, there are a number of considerably different techniques. Several methods of casting a cellular material are similar to investment casting. In one method, a foamed plastic, having interconnecting pores, is filled with a fluidized refractory material which is subsequently hardened. Upon heating and vaporizating the plastic, a spongy, skeletal mold is produced. A melt is then poured into the mold and, after solidification, a cellular structure is obtained. This method has particular application with metals having low melting points.
A mold for producing a porous material with a high melting point can be made by compacting an inorganic powder material, which is soluble in at least one solvent, to form a porous solid having interconnected powder particles. The molten material is then introduced into the pores of the mold where it solidifies. After cooling, the inorganic material is removed by the solvent.
Another technique involves a mold filled with granules. When the molten material is poured in the mold, the material penetrates into the voids between the granules and an interconnected cellular structure will be produced once the granules have been removed. The technique required for removing the granules will depend upon the specific granules utilized.
A mechanical method which produces a controlled pore structure involves a mold having opposing plates with pins protruding into the mold cavity. After a molten metal has been injected and solidified, the plates are moved apart and the pins removed providing the casting with its pore structure.
Foaming techniques have also been seen. According to these methods, a foaming agent is added to a molten metal and the resulting foam is cooled to form a solid of foamed metal. Typical foaming agents include hydrides, silicon, aluminum, sulphur, selenium and tellurium among others.
A limitation of the foaming process is that the size and distribution of the pores can only be controlled to a very limited extent. Another limitation of the foaming techniques which makes casting very difficult is the short time interval involved between adding the foaming agent and foam formation. Additional difficulties are caused by the premature decomposition of the foaming agent. If nonporous sections are desired within the casting, barrier layers must be provided producing additional difficulties. Thickening agents have been used in an attempt to control pore formation. However, these agents often produce negative effects with regard to the mechanical properties of the foamed metal.
Solutions to overcome the foregoing problems have been proposed which involve blowing bubbles of an inert gas into the molten material while the material concurrently solidifies. As such, the gas being blown into the melt causes the formation of hollow, semi-molten metal granules which become bound together to form a cellular type structure.
Review of the above methods for manufacturing porous materials shows that their common disadvantage lies primarily in their complexity. This complexity arises due to the necessity of involving a considerable number of operations and/or using a considerable number of preparatory stages. As a direct result, the cost of the produced product is high and the production rate is low. Both of which make the resulting material commercially impractical.
With the above limitations in mind, it is accordingly the primary object of the present invention to provide a simplified process for manufacturing porous articles, including pure metals, alloys and ceramics.
Another object of the invention is to provide a process which allows for predetermined sizes, shapes and orientations of pores within the article, as well as allowing for the formation of adjacent porous and nonporous regions.
The above objects are achieved as a result of the discovery of the in situ formation of pores during the decomposition of a liquid which is accompanied by the simultaneous occurrence of a crystalline phase and a gaseous phase. According to the present invention, a base material (metal, alloy or ceramic) is melted within an autoclave in an atmosphere of a gas, containing hydrogen, under a specified pressure. The melt is exposed to the gas for a period of time such that the hydrogen is dissolved therein and its concentration within the melt has reached a prescribed saturation value. This operation is hereinafter referred to as saturating.
After saturating, the melt (now containing the dissolved hydrogen gas therein) fills a mold also positioned within the autoclave. Immediately after filling, the pressure within the autoclave is set to a prescribed level and the melt is cooled. The pressure at which the melt is cooled is hereinafter referred to as the solidification pressure.
As the saturated melt solidifies, the solubility of the dissolved gas displays a sharp decrease. The quantity of gas which represents the difference between the gas content dissolved in the melt and the amount which is soluble in the solidified material evolves in the form of gas bubbles immediately ahead of the solidification front. The gas bubbles grow concurrently with the solid and do not leave the solidification front thus, forming the cellular structure.
The solidification pressure will be controlled after pouring depending on the desired pore size, pore structure and void content. If a porous article exhibiting cylindrical pores is desired, the solidification pressure is held constant until solidification has been completed and the heat flow through the article is controlled. If a more intricate pore structure is desired (e.g. tapered, ellipsoidal or spherical pores) the solidification pressure is accordingly increased or decreased during solidification. If a nonporous region is desired in the resulting product, the solidification pressure is significantly increased above an upper pressure limit after which pore formation will not occur.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims taken in conjunction with the accompanying drawings.